I remember that this was the argument of a discussion I had with a collegue about an year ago. He bought the Vodafone UMTS Connect Card and I said that could be a good idea if an hardware manufacturer a day will embed the UMTS connectivity into its laptop models.
Now seems that this is become reality: Dell is planning to embed Vodafone's 3G wireless broadband technology in notebook PCs. 
This new way of connectivity permits you to have a supplemental wireless broadband connectivity, especially for who needs extreme mobility, who is on travel and may not have the chance to have a WiFi hotspot available.
Maybe Dell has opened a new road? I hope so...